Electrochemical cells are presently the preferred method of providing cost effective portable power for a wide variety of consumer devices. The consumer device market dictates that only a handful of standardized cell sizes (e.g., AA or AAA) and specific nominal voltages (typically 1.5 V) be provided. Moreover, more and more consumer electronic devices, such as digital still cameras, are being designed with relatively high power operating requirements. As has been the practice within the market, consumers often prefer and opt to use primary batteries for their convenience, reliability, sustained shelf life and more economical per unit price as compared to currently available rechargeable (i.e., secondary) batteries.
Within this context, it is readily apparent that design choices for primary (i.e., non-rechargeable) battery manufacturers are extremely limited. For example, the necessity of using specified nominal voltages significantly limits the selection of potential electrochemical materials, and the use of standardized cell sizes restricts the overall available internal volume available for active materials, safety devices and other elements typically expected in such consumer products. What's more, the variety of consumer devices and the range of operating voltages for those devices make smaller nominal voltage cells (which can be provided separately or in series, thereby giving device makers more design options) more versatile as compared to higher voltage electrochemical pairings typically associated with secondary batteries. Thus, 1.5 V systems, such as alkaline or lithium-iron disulfide systems, are far more prominent than others, such as 3.0 V and higher lithium-manganese dioxide.
Within the realm of 1.5 V systems, lithium-iron disulfide batteries (also referred to as LiFeS2, lithium pyrite or lithium iron pyrite) offer higher energy density, especially at high drain rates, as compared to alkaline, carbon zinc or other systems. However, current regulatory limitations on the amount of lithium (the preferred electrochemically active material in the anode) make the FR03 (AAA LiFeS2 cells) and FR6 (AA LiFeS2 cells) sizes the most significant cell sizes for this chemistry within the consumer market.
The design considerations for 1.5V electrochemical systems (e.g., alkaline v. lithium-iron disulfide, etc.) are significantly different. For example, alkaline and nickel oxy-hydroxide systems rely on an aqueous and highly caustic electrolyte that has a propensity for gas expansion and/or leakage, leading to very different approaches in terms of selection of internal materials and/or compatibility with containers and closures. In rechargeable 1.5 V systems (note that lithium-iron disulfide systems are not currently considered suitable for consumer-based rechargeable systems), various highly specialized electrochemical and/or electrolyte compositions may be used to best accommodate lithium ion charge/discharge cycling. Here, such high cost components are not a key design concern because secondary systems typically sell for a higher retail price than their primary battery equivalents. Moreover, the discharge mechanisms, cell designs and safety considerations are, by and large, inconsequential and/or inapplicable to primary systems.
Improvements to capacity represent a fundamentally sound battery design. That is, in order to deliver greater capacity, careful consideration must be given for the radial expansion forces and other dynamics at work in a discharging lithium-iron disulfide battery. For example, if the design provides inadequate thickness in the anode or the cathode current collector then the radial forces during discharge may compress the jellyroll to such a degree so as to cause a disconnect in one or both electrodes and, once this disconnect occurs, the battery may cease to deliver capacity regardless of whether the active materials have all been discharged. Similar situations arise with respect to the void volume (in the cathode coating and the interior of the cell as a whole), the electrical connections throughout the battery, the separator, the closure/venting mechanism for the battery and the like. Therefore, the capacity of a LiFeS2 cell is a significant metric for the overall viability and robustness of a cell design, particularly when the cell designer is limited to the use of a standard-sized consumer battery (e.g., AA or FR6; AAA or FR03; etc.)
As a corollary to the capacity acting as a de facto metric for battery design, those skilled in the art will appreciate that design choices, and particularly the selection of specific components, must be made in consideration of the overall battery. A specific composition may have surprising, unexpected or unintended effects upon the other components and compositions within the cell. Similarly, in standard sized batteries, the selection of a particular element occupies volume within the container that might otherwise have been available for other elements. Thus, this interdependency of design choices necessarily means that any increase in capacity, and especially an increase that does not negatively impact the safety or performance of the battery in other regards, is much more than a simple act of adding more active materials.
Yet another important consideration for cell designers in LiFeS2 systems relates to minimizing the internal resistance of the cell. Generally speaking, the internal resistance is caused by the components used to make the cell, and can be expressed as follows:Rcell=Rcontainer+Relectrode assembly 
The resistance from the container components (Rcontainer) includes resistance caused by the can (including external contact terminals), internal electrical connections (e.g., welds or pressure contacts), internal safety devices (e.g., PTC) and the like. Typically, the resistance from these container components will behave in a relatively predictable and easy to control manner, thereby making it relatively simple to minimize this contribution.
However, the resistance caused by the electrode assembly (Relectrode assembly) can be an indicator of the overall quality of the design because this resistance is much more difficult to predict and control. Moreover, in a lithium cell where the anode consists essentially of highly conductive lithium or a lithium-based alloy, the resistance of the electrode assembly will depend and vary almost entirely upon the selection of the separator and the cathode. Thus, how and what is coated onto the cathode current collector, in conjunction with selection of an appropriate separator, can be viewed as having a direct, measurable effect on the overall resistance of a cell. Extending this concept one step further, in a series of cells where the components of the container and the separator are essentially identical, the overall resistance of the cell will serve as an excellent proxy of comparison as to the desirability of the cathodes for those cells.
Even with the inherent advantages of lithium-iron disulfide cells for high power devices (as compared to primary alkaline cells), LiFeS2 cell designs must strike a balance between the cost of materials used, the incorporation of necessary safety devices and the overall reliability, delivered capacity and intended use of the designed cell. Normally, low power designs emphasize the quantity of active materials, while high power designs focus more on configurations to enhance discharge efficiency. For example, a jellyroll design maximizes the surface area between the electrodes and allows for greater discharge efficiencies, but in doing so, might sacrifice capacity on low power and low rate discharges because it uses more inactive materials, such as separator and current collector(s) (both which occupy internal volume, thereby requiring removal of active materials from the cell design).
In addition to improved capacity, cell designers must also consider other important characteristics, such as safety and reliability. Safety devices normally include venting mechanisms and thermally activated “shutdown” elements, such as positive thermal circuits (PTCs). Improvements to reliability primarily focus on preventing internal short circuits. In both instances, these characteristics ultimately require elements that occupy internal volume and/or design considerations that are usually counterproductive to cell internal resistance, efficiency and discharge capacity. Moreover, there are additional challenges because transportation regulations limit the percent amount of weight lithium batteries can lose during thermal cycling, meaning that cell designs for smaller container sizes like AA and AAA can only lose milligrams of total cell weight (usually by way of evaporation of the electrolyte). Plus, the reactive and volatile nature of the non-aqueous, organic electrolyte severely limits the universe of potential materials available (particularly with respect to interactions between the electrolyte and cell closure, separator and/or current collector(s) provided within the cell) as compared to other electrochemical systems.
Ultimately, maximizing the amounts of active materials in lithium-iron disulfide batteries while maintaining optimal properties, particularly with respect to the cathode, may be the most difficult challenge. As noted above, the jellyroll electrode assembly is the preferred configuration in LiFeS2 systems. In order to accommodate iron disulfide most effectively, the iron disulfide is mixed into slurry with conductors and binders and then coated onto a metallic foil current collector, while the lithium is most effectively provided without a current collector. Lastly, the separator is a thin polymeric membrane whose thickness is preferably minimized to reduce the inactive inputs into the cell.
Because the reaction end products occupy more volume than the inputs, the electrode assembly swells as the battery discharges. In turn, swelling creates radial forces that can cause unwanted bulging of the cell container, as well as short circuits if the separator is compromised. Previous means of handling these problems include using strong (often thicker) materials for the cell housing and inactive components within the cell. However, thicker inactive materials limit the internal volume available and thicker, more rugged electrodes were previously deemed not necessarily desirable because they allow for fewer winds possible in the jellyroll, resulting in less surface area between the electrodes and the expectation of comparatively lower performance at higher drain rates.
A number of other approaches have been taken to strike an appropriate balance between optimal internal volume utilization and acceptable LiFeS2 cell capacity/performance. For example, a possible solution for problems created by swelling, disclosed in U.S. Pat. No. 4,379,815, is to balance cathode expansion and anode contraction by mixing one or more other active materials (such as CuO, Bi2O3, Pb2Bi2O5, P3O4, CoS2) with pyrite, although these additional materials can negatively affect the discharge characteristics of the cell, and the capacity and efficiency of the overall cell may also suffer.
Other means of improving discharge capacity in LiFeS2 cell contemplate the use of thinner separators and/or specific cathode coating techniques and pyrite particle sizes, as respectively disclosed in U.S. Patent Publication Nos. 20050112462 and 20050233214.
U.S. Pat. Nos. 6,849,360 and 7,157,185 discloses the use of a specific cathode coating formulation and an anode provided as pure lithium (or a lithium-aluminum alloy) to obviate the need for an anode current collector. The amount of anode and cathode are then provided at specified ratio of anode to cathode interfacial active materials (i.e., the theoretical interfacial input capacity ratio) in order to enhance LiFeS2 cell high rate performance.
U.S. Patent Publication Nos. 20090070989 and 20080050654 and Chinese Patent Publication Nos. 1845364 disclose cathode formulations have 95 wt. % or less of FeS2 that may be pertinent to cathode coatings for electrodes in LiFeS2 cells. United States Patent Publication No. 20070202409 and Chinese Patent Publication Nos. 1790781 disclose cathode formulations having at least 3 wt. % of binders that may also be pertinent to cathode coatings for electrodes in LiFeS2 cells. Chinese Patent Publication No. 1564370 generically discloses a mixture of pyrite, binders and conductors that may be pertinent to LiFeS2 cells.